389-ds-base/lib/base/rwlock.cpp

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 * Copyright (C) 2001 Sun Microsystems, Inc. Used by permission.
 * Copyright (C) 2005 Red Hat, Inc.
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 * END COPYRIGHT BLOCK **/
#include <stdlib.h>
#include "crit.h"
#include "rwlock.h"

/*
 * rwLock.c
 *    Implements a shared/exclusive lock package atop the
 * critical section/condition variables. It allows multiple
 * shared lock holders and only one exclusive lock holder
 * on a lock variable.
 *
 * NOTE : It currently favors writers over readers and writers
 *  may starve readers. It is usually preferable to allow updates
 *  if they are not as frequent. We may have to change this if
 *  the common usage pattern differs from this.
 */

typedef struct {
    CRITICAL crit;           /* Short term crit to synchronize lock ops */
    CONDVAR  readFree;       /* Indicates lock is free for readers */
    CONDVAR  writeFree;      /* Indicates lock is free for the writer */
    int      numReaders;     /* Number of read locks held */
    int      write;          /* Flag to indicate write lock held */
    int      numWriteWaiters;/* Number of threads waiting for write lock */
} rwLock_t;

/* 
 * rwlock_init()
 *   Allocate and initialize the rwlock structure and return
 *  to the caller.
 */
RWLOCK rwlock_Init()
{
    rwLock_t *rwLockP;

    rwLockP = (rwLock_t *)PERM_MALLOC(sizeof(rwLock_t));
    rwLockP->numReaders = 0;
    rwLockP->write = 0;
    rwLockP->numWriteWaiters = 0;
    rwLockP->crit = crit_init();
    rwLockP->readFree = condvar_init(rwLockP->crit);
    rwLockP->writeFree = condvar_init(rwLockP->crit);
    return((RWLOCK)rwLockP);
}

/* 
 * rwlock_terminate()
 *   Terminate the associated condvars and critical sections
 */
void rwlock_Terminate(RWLOCK lockP)
{
    rwLock_t   *rwLockP = (rwLock_t *)lockP;
    
    crit_terminate(rwLockP->crit);
    condvar_terminate(rwLockP->readFree);
    condvar_terminate(rwLockP->writeFree);
    PERM_FREE(rwLockP);
}

/*
 * rwlock_ReadLock -- Obtain a shared lock. The caller would
 *  block if there are writers or writeWaiters.
 */
void rwlock_ReadLock(RWLOCK lockP)
{
    rwLock_t *rwLockP = (rwLock_t *)lockP;

    crit_enter(rwLockP->crit);                 
    while (rwLockP->write || rwLockP->numWriteWaiters != 0)
        condvar_wait(rwLockP->readFree);
    rwLockP->numReaders++;                              
    crit_exit(rwLockP->crit);                 
}

/*
 * rwlock_writeLock -- Obtain an exclusive lock. The caller would
 * block if there are other readers or a writer.
 */
void rwlock_WriteLock(RWLOCK lockP)
{
    rwLock_t *rwLockP = (rwLock_t *)lockP;

    crit_enter(rwLockP->crit);                 
    rwLockP->numWriteWaiters++;                                
    while (rwLockP->numReaders != 0 || rwLockP->write)             
        condvar_wait(rwLockP->writeFree);      
    rwLockP->numWriteWaiters--;                                
    rwLockP->write = 1;                                   
    crit_exit(rwLockP->crit);                 
}

/*
 * rw_Unlock -- Releases the lock. 
 */
void rwlock_Unlock(RWLOCK lockP)
{
    rwLock_t *rwLockP = (rwLock_t *)lockP;
                                                         
    crit_enter(rwLockP->crit);                 
    if (rwLockP->write)                                      
        rwLockP->write = 0;                              
    else                                                 
        rwLockP->numReaders--;                                 
    if (rwLockP->numReaders == 0)                              
        if (rwLockP->numWriteWaiters != 0)                     
            condvar_notify(rwLockP->writeFree);             
        else                                             
            condvar_notifyAll(rwLockP->readFree);           
    crit_exit(rwLockP->crit);                 
}
                                                         
/*
 * rwlock_DemoteLock -- Change an exclusive lock on the given lock
 * variable into a shared lock.  
 */
void rwlock_DemoteLock(RWLOCK lockP)
{
    rwLock_t *rwLockP = (rwLock_t *)lockP;
                                                         
    crit_enter(rwLockP->crit);                 
    rwLockP->numReaders = 1;
    rwLockP->write = 0;
    if (rwLockP->numWriteWaiters == 0)
        condvar_notifyAll(rwLockP->readFree);
    crit_exit(rwLockP->crit);                 
}